The invention is directed to thrust bearing for pumps and turbines. More particularly, the thrust bearing of the present invention is designed to accommodate axial thrust loads. In pumps and turbines, there is usually an imbalance in the pressure exerted by the pumped fluid on the rotating impeller. At the inlet for fluid, the pressure is low and usually the pressure from the pumped fluid on the opposite side of the impeller is high. This creates a pressure imbalance on the impeller that can affect the operation of the pump or turbine.
Axial thrust loads on the rotors of pumps and turbines is a universal occurrence. Sometimes axial thrust is balanced symmetrically or nearly so, on either side of the rotors, as in the case of double suction pumps, so that thrust absorption of the rotor is not a significant problem. However, in the majority of designs significant net axial thrust is imposed on the impeller to the extent that provisions are made to incorporate thrust bearings into the pump or turbine.
In the majority of designs of industrial centrifugal turbo machines the thrust bearing is located in a housing separate from the pressure casing. These bearings are usually of either the rolling contact type (anti friction ball and roller bearing) or of the sliding contact type (hydrostatic or hydrodynamic) and are lubricated by grease or oil.
A centrifugal pump or turbine design can be greatly simplified by positioning the thrust bearing in the pressure casing as described in U.S. Pat. No. 5,082,428. Such an arrangement not only absorbs axial thrust loads but just as significantly provides an exceptionally efficient face seal between the high pressure region of the pump (discharge) and the low pressure region (section). Volumetric efficiencies of 98 to 99 percent have been achieved in practice. This compares to typical wearing ring equipped pumps having volumetric efficiencies (as new) of 0.85% to 0.9%. However, as the wear rings become worn, the efficiency of the pump decreases.
The present invention relates to and is an improvement of the high speed centrifugal pump described previously in U.S. Pat. No. 5,082,428 issued on Jan. 21, 1992 and is also applicable to reverse running pump turbines of similar design. Specifically, the invention is a new and useful improvement in the lubricated thrust bearing/seal originally described in the above patent. The thrust bearing is a hydrostatic bearing positioned adjacent to the suction side of the pump impeller. Both the impeller face and the bearing face are flat and parallel to each other and precisely perpendicular to the axis of rotation. A 360.degree. annular groove is positioned at the working face of the thrust bearing/seal. The inner land provides a seal between the annular groove and the low pressure area of the pump. The outer land of the thrust bearing provides a seal between the annular groove and the high pressure region of the pump.
The annular groove is in communication with the highest pressure region of the pump, the diffuser, by means of a conduit. For operation of original invention see column 4, line 21 through column 6, line 25.
It has been found in practice that the purely hydrostatic thrust bearing/seal works well in cases where the impeller OD to impeller wearing ring OD ratio is sufficiently large to provide a large enough annular groove area along with sufficient land area for effective sealing. This is true of low specific speed impellers (i.e., low flow and high head). However, with higher specific speed impellers (high flow/low head) that have relatively large wearing ring (suction eye) diameters and a relatively small impeller OD, there is only a marginal hydrostatic area available for thrust balancing. Hence, high speed sliding contact could occur in high specific speed impellers often resulting in thrust bearing failure.
Another drawback to the originally described thrust bearing/seal was the provision of supplying pressured fluid from the diffuser section of the pump. In operation, the pressure in the diffuser section is sufficient for hydrostatic thrust bearing operation when the pump was operating in the capacity range of between shutoff (zero flow) and best efficiency point (BEP). At capacity greater than BEP (run out condition) the fluid velocity in the diffuser increases to a point where the static pressure in the conduit falls below the annular groove pressure resulting in reverse flow in the conduit. Such reverse flow means there was insufficient hydrostatic pressure in the annular groove to prevent heavy sliding contact between impeller face and thrust bearing face-end resulting in a destroyed thrust bearing/seal.
Run-out conditions are often present at the startup of a pumping system when the system is being filled. Air is being displaced through throttle devices such as valves or orifices with very little pressure resistance (due to low density of air in comparison to most fluids such as water) thereby causing the centrifugal pump to operate at much greater than BEP capacity. Even if such operation is only for a few seconds, enough rubbing contact can occur in high speed pumps to cause thrust bearing failure. Also careless operation of pumps beyond original design conditions is quite common. The present invention overcomes these problems.
Accordingly, it would be desirable to have an axial thrust bearing for pumps and turbines that balances the axial loads on the impeller and allows the pump or turbine to operate as efficiently as possible.